Smart electric heater for bathing installations

Abstract

A water heating system for a bathing installation, including a heating element in a housing in a water flow path of the bathing installation. A switching system selectively connects the heating element to an AC power connection. A heater control system controls the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontroller configured to control the switching system to control current flow to the heating element to control the apparent heater element resistance and optimize power usage regardless of the current and voltage applied to the heating system from the AC power connection. In one embodiment, a monitoring circuit monitors the heating element impedance, and the microcontroller detects dry fire conditions in dependence on changes in the impedance. The heating system may include a flow measuring system for monitoring water flow through the housing.

Description

SMART ELECTRIC HEATER FOR BATHING INSTALLATIONSBACKGROUND OF THE INVENTION

[0001] This application claims priority to provisional Application Number 63 / 707,879, filed October 16, 2024.

[0002] This invention is directed to heating systems for heating water in bathing installations. Exemplary heating systems known in the art are described in US Patents 7,030,343 and 8,396,356, the entire contents of which are incorporated herein by this reference.BRIEF DESCRIPTION OF DRAWINGS

[0003] FIG. 1 is a schematic block diagram of an exemplary embodiment of a heating system in accordance with aspects of the invention.

[0004] FIG. 2 illustrates an exemplary trip curve for a circuit breaker.

[0005] FIG. 3 illustrates TRIAC modulation (full cycle). FIG. 3A illustrates an exemplary circuit schematic for implementing full cycle modulation.

[0006] FIG. 4 illustrates TRIAC modulation (phase cycle). FIG. 4A illustrates an exemplary circuit schematic for implementing TRIAC modulation (phase cycle).

[0007] FIG. 5 illustrates TRIAC modulation (gate turn off).

[0008] FIG. 6 illustrates an exemplary rectified AC voltage. FIG. 6A illustrates pulse width modulation (PWM) of current using a MOSFET. FIG. 6B is an exemplary circuit schematic for implementing PWM current control to a heating element.

[0009] FIG. 7 illustrates one exemplary implementation of two thermistors located in a probe which can be immersed in the water or other fluid. FIG. 7A depicts exemplary embodiments of circuit schematics for implementing a self-heating thermistor flow sensor circuit, a thermistor temperature sensor circuit and a comparator circuit.

[0010] FIG. 8 illustrates an exemplary embodiment of a heater element resistance monitoring circuit 70.

[0011] FIGS. 9A-9D illustrate a heater system utilizing localized heating with temperature sensing to measure fluid flow rates through a tube.DETAILED DESCRIPTION OF THE INVENTION

[0012] System Overview:

[0013] In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. The figures may not be to scale, and relative feature sizes may be exaggerated for illustrative purposes.

[0014] FIG. 1 is a schematic block diagram illustrating an exemplary embodiment of a heating system configured for heating water in a bathing installation, such as a spa, swim spa or whirlpool bath. This embodiment controls the application of AC power to a resistive heating element 20 which is configured to heat water pumped through a recirculating water flow path. Examples of bathing installations suitable for employment of this invention are described in US Patents 6,282,370 and 8,669,494, the contents of which are incorporated herein by this reference.

[0015] The heating control circuit 50 includes microcontroller 100, and controls high voltage / current 32 entering the system from the system interface 30 on main lines 34, 36, regulating the AC power applied to the heating element 20 by two TRIACs 62, 64. While a single TRIAC could be used, a redundant one is included for safety. These TRIACs can be replaced with other types of electronic switches as described below. Current is monitored by current sensor 66 from these main lines 34, 36 for power consumption and monitoring applications. Additionally, the impedance of heating element 20 is measured by impedance monitoring circuit 70 for purposes described below. The heating control circuit 50 may be mounted, in an exemplary embodiment, on a printed circuit board 156 mounted to the heating element tube or housing 22 (FIGS. 9A, 9C).

[0016] In an exemplary embodiment, the heater assembly includes the control circuit 50 responsible for monitoring flow and temperature (described more fully below), as well as handling basic analog safety limits. In this embodiment, the control circuit is attached to the heater tube. A bimetal switch 120 is used for temperature limiting and control. The system features zero-crossing detectionthrough zero-crossing detection circuit 102 to ensure that the switching circuits (including TRIACs 62, 64) are precisely synchronized with the AC waveform, preventing circuit breakers from tripping. The control circuit includes temperature sensors 130, 140 in one embodiment with the temperature sensors mounted in a probe extending into the water within the heater housing or tube, and associated excitation and amplifier circuits 130A, 130B, 140A, 1406. In a second embodiment, the temperature sensors 152, 154 are mounted along the tube surface, as discussed below regarding FIGS. 9A-9D.

[0017] This exemplary embodiment of a smart heater also includes communication capabilities, allowing it to be commanded by a user interface (III) or other control devices. In this exemplary embodiment, a RS485 transceiver 106 is connected to a control interface 36 of the system interface 30. This provides communication between the control interface and an authentication function 100- A implemented in this embodiment by the microcontroller 100. The authentication function is provided to ensure that only authentic heater systems are under control of the III. An exemplary authentication method and system is described in US Patent Application 18 / 368,560, filed September 14, 2023, the entire contents of which application is incorporated herein by this reference. Opto-isolation between the TRIACs and the microcontroller 100 is implemented by opto-isolators 104A, 104B to ensure that connections to external displays or devices are safe for the user.

[0018] Heating Control:

[0019] Heating elements used in water heating applications are generally simple devices that act as resistors, enclosed in a tube placed in the water path. The resistance value of these elements is fixed, meaning that traditionally, different heating elements would need to be installed to accommodate varying voltages and currents at different installation sites. However, in a universal input heater in accordance with an aspect of this invention, a single resistance heating element 20 of a given power rating (e.g. one rated at 6.5 kW), and with a nominal resistance (9.97 -11 .64 ohms in this example) is used, and electronic controls are incorporated to prevent circuit breakers from tripping. These controls tightlymanage when the heating elements are energized to stay within the limits of the trip curves. FIG. 2 illustrates an exemplary trip curve for a circuit breaker.

[0020] The electronic control 50 switches the heating element 20 using one, or a combination, of the methods described below, making the fixed resistance element appear as though it has different resistances. For example, a 2kW heater switched at a 50% duty cycle appears as a 1 kW heater. This allows the system to control the apparent resistance and optimize power usage, regardless of the current and voltage available at the installation site. Switching a heater element at different duty cycles allows the same heating element to be used across different current and voltage input conditions, such as 120VAC and 240VAC. This capability is tied to the control over the apparent resistance of the heater. For example, a 5kW heater designed for 120V would require a resistance of approximately 2.9 Ohms. However, for the same 5kW output on a 240V circuit, the resistance needs to be around 1 1 .5 Ohms. By adjusting the duty cycle to around 25%, the heater’s 2.9 Ohm element can behave as though it has an apparent resistance of 1 1 .5 Ohms, allowing it to operate safely and efficiently at 240V. Similarly, a 2.9 Ohm heater on a 120V circuit would require a 40 Amp circuit breaker to run at full capacity. By regulating the duty cycle to 50%, the power output is reduced, allowing the heater to operate on a 20 Amp circuit, though at a lower total power output. This flexibility allows the heater to adapt to different input conditions while maintaining safe operation across varying voltage and current levels. The electronic controller 50 knows the AC power connection, through current sensor 66, heater impedance monitoring circuit 70, and application of Ohm’s law to calculate the voltage.

[0021] This flexibility is achieved by exploiting the time component of circuit breakers, momentarily exceeding the trip current but de-energizing the circuit before the breaker trips. In the 120V example, the current drawn by the heater can momentarily exceed the 20 Amp limit without causing the circuit breaker to trip, due to the inherent time-delay feature of standard circuit breakers. This time component is described by the circuit breaker's trip curve, which defines how long a circuit can exceed its rated current before the breaker trips. Circuitbreakers are designed to tolerate brief overcurrent conditions without immediately disconnecting power. For instance, a 20 Amp breaker may allow currents above 20 Amps for a short duration before tripping, depending on the severity of the overcurrent and the time it persists. By carefully controlling the duty cycle of the heater, the system can exploit this trip curve. The current may exceed 20 Amps momentarily, but it is switched off before the time threshold defined by the trip curve is reached. This prevents the breaker from tripping, allowing the heater to operate safely under these conditions without needing to upgrade the breaker or wiring.

[0022] TRIAC Modulation (Full cycle)- First Exemplary Heating Control Method

[0023] A TRIAC is a circuit element commonly used to rapidly switch AC power systems on and off. To minimize average power consumption and prevent tripping circuit breakers, detection circuit 102 detects the zero crossings of the waveforms, allowing for the precise control of full AC cycles. This is desirable from an EMI / EMC compliance standpoint and generally better for power generation and distribution systems. FIG. 3 illustrates TRIAC modulation (full cycle). FIG. 3A illustrates an exemplary circuit schematic for implementing full cycle modulation.

[0024] TRIAC Modulation (Phase Control)

[0025] Utilizing phase control with TRIACs offers similar average current reductions as using full cycle modulation, but provides greater control over peak currents. However, a drawback is that they can only be turned off at zero crossing, limiting the dynamic range of the current limiting capability. In other words, if the current needs to be limited by the peak demands, the average may also be reduced by more than necessary based on the circuit protection device. FIG. 4 illustrates TRIAC modulation (phase cycle). FIG. 4A illustrates an exemplary circuit schematic for implementing TRIAC modulation (phase cycle).

[0026] TRIAC Modulation (Gate Turn Off)

[0027] Gate turn-off (GTO) TRIACs offer the highest level of control, enabling full management of both average current draw and the elimination of high peak currents. This allows for the utilization of the largest dynamic control range.Consequently, the maximum power can be delivered to the heating element, with both the average current and peak current maintained below the trip levels for the given circuit protection systems. FIG. 5 illustrates TRIAC modulation (gate turn off).

[0028] H-bridge Modulation (MOSFET)

[0029] MOSFETs serve similar purposes to TRIACs but offer the added benefit of being able to switch on and off rapidly at any time. However, they are not suitable for switching AC power directly. To address this, AC power can be converted using a full bridge rectifier. FIG. 6 illustrates an exemplary rectified AC voltage.

[0030] Following this conversion, a MOSFET can regulate current through PWM (pulse width modulation), illustrated in FIG. 6A. To prevent ion migration, an H- bridge configuration is employed, allowing the heating element to experience AC current while isolating the electronics from it. FIG. 6B is an exemplary circuit schematic for implementing this form of power control to the heating element.

[0031] Flow and Liquid Presence Sensing:

[0032] Self-Heating Thermistor

[0033] A thermistor is a type of temperature measuring device that changes its resistance based on the temperature of its surroundings. Typically, these resistive elements undergo a minimal amount of self-heating to maintain accurate temperature readings. However, by designing the circuit to amplify the selfheating of the thermistor, it can be utilized to measure both the presence of water and, with a known tube diameter, the velocity (and therefore the flow rate) of the water.

[0034] This can be achieved by applying power to the thermistor and comparing the rate of change of its resistance. A slower change corresponds to flowing water, as the water's movement increases the cooling effect (similar to wind chill). Alternatively, the steady-state temperature of the thermistor can be compared to a second thermistor not designed for self-heating to derive the water flow rate.

[0035] FIG. 1 shows exemplary thermistor 140 used as a liquid presence sensor, with an excitation circuit 140 A to power the thermistor, and an amplifier 1406connected to the thermistor to amplify the thermistor measurement and provide the amplified measurement signal to the microcomputer 100. A second thermistor 130 is also used as an outlet temperature probe, with excitation circuit 130A and amplifier 130B to send a measurement signal to the microcomputer, providing the actual water temperature within the heater tube. In other embodiments, the amplifier circuits 140B and 130B may be omitted. The thermistor signals may only need to be amplified to make them readable and accurate by the microcontroller but may also be achievable by using a larger excitation voltage.

[0036] FIG. 7 illustrates one exemplary implementation of two thermistors 130, 140 located in a probe 132 which can be immersed in the water or other fluid, typically within the heater 20 tube. In this embodiment, the thermistors are mounted in thermally conductive epoxy within the probe. FIG. 7A depicts exemplary embodiments of circuit schematics for implementing a self-heating thermistor flow sensor circuit, a thermistor temperature sensor circuit and a comparator circuit responsive to the outputs of the flow sensor circuit and the temperature sensor circuit. In this exemplary embodiment, the circuit output is a logic level signal that indicates whether the system detects the right amount of flow ( we need a temperature rise of less than a predetermined amount) so it is either high or low based on the voltage input.

[0037] Dry Fire Protection:

[0038] Heating Element Resistance Monitoring

[0039] Dry firing or activating the heating element without the presence of water can pose both safety risks and potential damage to the heating element, leading to premature failure. To prevent such occurrences, most circulation heaters are equipped with either a liquid presence sensor or a flow sensor to ensure the heating element is submerged in liquid before activation. However, adding additional components to the tubing system is less than ideal.

[0040] Resistive heating elements such as heating element 20 exhibit a slight change in resistance as their temperature varies. Consequently, a circuit designed to measure the resistance of the heating element — e. g. by monitoringthe current and voltage across it using Ohm's Law (V=IR) or by conducting a standard resistor divider measurement during an off period when not applying 120V or 240V to the circuit (made possible due to the heating control methods described above) — can detect an over-temperature condition caused by a dry fire event and subsequently disable the element.

[0041] In an exemplary embodiment shown in FIG. 8, the resistance monitoring circuit 70 utilizes a Colpitts oscillator for measurement purposes in a heating system, where the oscillator's frequency varies with the resistance of the heating element. The series capacitors in the circuit are configured as part of a high- pass filter. Their purpose is to attenuate the high voltage AC (50 / 60 Hz) signal used to energize the heater while allowing the higher frequency oscillator signal to pass through. The output of circuit 70 in an exemplary embodiment is a logic level square wave, whose frequency is measured by the microcontroller 100.

[0042] By monitoring the oscillator frequency, which correlates with the resistance of the heating element, the temperature of the heating element can be determined continuously, providing real-time feedback even while the heater is operational. Both the steady-state temperature and the rate of change of temperature provide useful information. Because the electrical energy applied to the resistive heating element is directly converted into heat, the amount of heat being generated is already known. By comparing this known heat input to the observed change in heater temperature, the effective thermal conductivity of the surrounding medium can be inferred. Air, water, and flowing water all exhibit different apparent thermal conductivities, and by modeling the relationship between applied power, heater temperature, and material temperature, the system can classify the medium as air, water, or water at different flow rates.

[0043] This approach also enables detection of dry fire conditions. Water acts as an excellent heat sink, efficiently absorbing and dissipating heat from the heating element. When the heater is submerged in water, the heat transfer is efficient, keeping the temperature rise slow and within expected limits. In contrast, in a dry fire condition, where the heating element is not surrounded by water, the lack of heat dissipation causes the temperature to increase much more rapidly and to amuch higher maximum level. By monitoring the heater’s impedance, which varies with temperature, the control system can detect the rapid temperature increase and elevated maximum temperature, signaling a dry fire condition and allowing the system to shut off the heater to prevent damage.

[0044] Flow Measurement Method for Water Heater Assembly

[0045] Flow measurement

[0046] Another exemplary embodiment of a flow measurement method used in the water heater assembly for bathing installations such as spas or swim spas combines localized heating with temperature sensing to measure fluid flow rates. As shown in FIG. 9A-9D and FIG. 1 , the system includes one or more local heating elements 150A, 150B, the main heating element 20, and two types of temperature sensors: a local temperature sensor 152 and a local reference temperature sensor 154. The local heating elements may be resistors, for example.

[0047] The local temperature sensor 152 is positioned within the local heating zone 160 of the water heater assembly in the tube 22. The sensor 152 measures the temperature of the fluid in the specific section where the local heating elements 150A, 150B are applied. This sensor 152 is located near or integrated with the local heating elements to capture the temperature increase of the fluid due to localized heating.

[0048] The local temperature sensor 152 provides data on the temperature rise of the fluid caused by localized heating due to heating elements 150A, 150B. This measurement helps in determining the heat added by the local heating elements and assessing the heating process.

[0049] The local heating elements 150A, 150B in this exemplary embodiment are responsible for heating specific sections of the fluid as it flows through the water heater. These elements are designed to provide localized heating to accurately measure flow rate based on the temperature change in the fluid.

[0050] The local heating elements 150A, 150B in this exemplary embodiment are positioned directly within the flow path of the water heater. They are strategically placed to ensure effective heating of the fluid in targeted areas. Thelocal heating elements create a controlled temperature rise in the fluid that is proportional to the time the fluid spends in the heated section. This localized heating effect allows for precise measurement of the fluid’s velocity based on the temperature differential between the heated and non-heated fluid.

[0051] The reference temperature sensor 154 is positioned upstream of the local heating section, and measures the temperature of the fluid before it enters the heated zone. This sensor 154 provides a baseline temperature measurement of the fluid that has not been affected by the local heating, i.e. the sensor provides a reference temperature for the fluid prior to local heating. This baseline is used to assess the temperature increase caused by the localized heating and to calculate the temperature differential.

[0052] The main heating element 20 heats the entire volume of water in the tube 22 more uniformly. This element is responsible for maintaining an overall consistent temperature for the water in the bathing installation but is not required for the flow measurement method. The main heater 20 operates independently of the local heating elements 150A, 150B used for flow measurement, and the flow measurement system works whether the main heater element 20 is energized or not.

[0053] The main heating element’s uniform heating of the water does not impact the localized heating required for the flow measurement method. The flow measurement relies specifically on the local heating elements to assess the fluid flow rate, while the main heater ensures the overall temperature consistency of the water in the system.

[0054] As fluid moves through the local heating zone 160, its temperature increases based on the time the fluid spends in the local heating zone. The faster the linear velocity of the fluid through the heating assembly tube, the less time it spends in the local heating section, resulting in a lower temperature increase within that zone. This temperature rise, measured by the local temperature sensor 152, is then compared to the temperature from the local upstream reference sensor 154. By comparing these temperatures, the absolutetemperature rise in the local heating zone can be determined independently of the temperature of the unheated fluid.

[0055] The temperature difference, reflecting the intensity of the local heating and the fluid’s velocity, is used to calculate the fluid’s linear velocity. This velocity is then related to the flow rate based on the cross-sectional area of the pipe or tube 22.

[0056] Flow Sensing Configuration

[0057] The flow sensing method also functions when the local heating zone and local temperature sensor are positioned upstream from the reference sensor. In this configuration, the temperature rise in the fluid is still measured in the local heating zone, and the temperature difference between the heated fluid and the downstream reference sensor allows the system to determine fluid velocity and flow rate effectively.

[0058] Although the foregoing has been a description and illustration of specific embodiments of the subject matter, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention.

Claims

AMENDED CLAIMSreceived by the International Bureau on 15 June 2026 (15.06.2026)Claims1. A water heating system for a bathing installation, wherein the bathing installation is one of a spa, swim spa or whirlpool bath, comprising:a resistive heating element disposed in a housing configured for placement in a water flow path of the bathing installation;a switching system configured to selectively connect the resistive heating element to main lines of an AC power connection;a current sensor for sensing current flow through the resistive heating element and generating a current sensor signal;an impedance monitoring circuit for monitoring the impedance of the resistive heating element and generating an impedance signal indicative of the resistance of the resistive heating element;a heater control system responsive to control signals received via a control interface to control the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontroller responsive to the current sensor signal and the impedance signal and configured to control the switching system to control current flow to the heating element.

2. A water heating system according to Claim 1 , wherein the microcontroller is responsive to the current sensor signal and the impedance to control current flow to the heating element to control the apparent heater element resistance and optimize power usage regardless of the current and voltage applied to the heating system from the AC power connection, and wherein the resistive heating element is operable for different AC power connections.

3. A water heating system according to Claim 2, wherein the microcontroller is configured to control the switching system to control an applied current to the heating element to momentarily exceed a circuit breaker trip current of a circuit breaker associated with the AC power connection and to de-energize the heating element before the circuit breaker trips.

4. A water heating system according to Claim 2, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to theAMENDED SHEET (ARTICLE 19)heating element, and the microcontroller is configured to modulate the TRIAC to achieve full cycle modulation.

5. A water heating system according to Claim 2, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to the heating element, and the microcontroller is configured to modulate the TRIAC to achieve phase control modulation.

6. A water heating system according to Claim 2, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to the heating element, and the microcontroller is configured to modulate the TRIAC using gate turn-off modulation.

7. A water heating system according to Claim 2, wherein the switching system includes a MOSSFET switch and H-Bridge, and the microcontroller is configured to modulate the current to the heating element by pulse width modulation (PWM).

8. A water heating system according to any of Claims 2-7, wherein the microcontroller determines the current and voltage of the power connection based on the current sensor signal and resistance of the resistive heating element.

9. A water heating system according to any of Claim 2-8, wherein the AC power connection is one of a nominal 120VAC service and a nominal 240VAC service.

10. A water heating system according to any preceding claim, wherein the microcontroller is responsive to the impedance signal from the monitoring circuit to detect dry fire conditions in dependence on changes in the impedance of the heating element.

11. A water heating system according to Claim 10, wherein the monitoring circuit includes an oscillator circuit coupled to the resistive heating element to provide an oscillator signal to the resistive heating element, and operable even while the resistive heating element is energized, wherein the frequency of the oscillator circuit varies with resistance of the resistive heating element.

12. A water heating system according to Claim 10 or 11 , wherein the oscillator circuit is coupled to the resistive heating element by a high pass filter configured toattenuate the high voltage AC signal while allowing the oscillator signal to pass through the filter.

13. A water heating system according to any of Claims 10-12, wherein the control system is configured to correlate the impedance of the resistive heating element to rapid temperature increase and higher maximum temperature, signaling a dry fire condition.

14. A water heating system according to any preceding, further comprising: a flow measuring system for monitoring water flow through the housing, the flow measuring system including one or more local heating elements for applying heat in a local heating zone within the housing under control of the microcontroller, a local temperature sensor for obtaining temperature readings within the local heating zone and a reference temperature sensor disposed upstream or downstream of the local heating zone within the housing, and wherein the microcontroller is responsive to sensor signals from the local and reference sensors to detect and measure water flow through the housing.

15. A water heating system according to Claim 14, wherein the one or more local heating elements are positioned directly within the flow path in the housing to create a controlled temperature rise in the water that is proportional to the time the water spends in the local heating zone, to allow for measurement of the water velocity based on the temperature differential between the heated and non-heated water.

16. A water heating system according to Claim 14 or Claim 15, wherein the one or more local heating elements comprise a resistor.

17. A water heating system according to any of Claims 14-16, wherein the heater control system is configured to operate the main resistive heating element independently of the flow measuring system.

18. A water heating system according to any preceding claim, furthercomprising:a liquid flow monitoring system for monitoring water presence and flow through the housing, the flow monitoring system including a self-heating thermistor disposed within a probe mounted in the housing, and an excitation circuit to power the self-heating thermistor, wherein the microcomputer is configured to process a thermistor measurement signal to detect flow through the housing.

19. A water heating system according to Claim 18, further comprising a second thermistor mounted within the probe, a second excitation circuit to power the second thermistor to send a second measurement signal to the microcomputer, providing the actual water temperature within the heater housing.

20. A water heating system according to Claim 18 or Claim 19, further comprising a self-heating thermistor flow sensor circuit including the selfheating thermistor, a thermistor temperature sensor circuit including the second thermistor, and a comparator circuit responsive to the outputs of the flow sensor circuit and the temperature sensor circuit to indicate whether flow is detected.21.A water heating system for a bathing installation, wherein the bathing installation is one of a spa, swim spa or whirlpool bath, comprising:a resistive heating element disposed in a housing configured for placement in a water flow path of the bathing installation;a switching system configured to selectively connect the resistive heating element to main lines of an AC power connection;a current sensor for sensing current flow through the resistive heating element and generating a current sensor signal;an impedance monitoring circuit for monitoring the impedance of the resistive heating element and generating an impedance signal indicative of the resistance of the resistive heating element;a heater control system responsive to control signals received via a control interface to control the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontrollerAMENDED SHEET (ARTICLE 19)responsive to the current sensor signal and the impedance signal and configured to control the switching system to control current flow to the heating element to control the apparent heater element resistance and optimize power usage regardless of the current and voltage applied to the heating system from the AC power connection.

22. The heating system of Claim 21, wherein the microcontroller is configured to control the switching system to control an applied current to the heating element to momentarily exceed a circuit breaker trip current of a circuit breaker associated with the AC power connection and to de-energize the heating element before the circuit breaker trips.

23. The heating system of Claim 22, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to the heating element, and the microcontroller is configured to modulate the TRIAC to achieve full cycle modulation.

24. The heating system of Claim 22, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to the heating element, and the microcontroller is configured to modulate the TRIAC to achieve phase control modulation.

25. The heating system of Claim 22, wherein the switching system includes a TRIAC connected to selectively connect one of the main lines to the heating element, and the microcontroller is configured to modulate the TRIAC using gate turn-off modulation.

26. The heating system of Claim 22, wherein the switching system includes a MOSSFET switch and H-Bridge, and the microcontroller is configured to modulate the current to the heating element by pulse width modulation (PWM).

27. The heating system of Claim21 , wherein the microcontroller determines the current and voltage of the power connection based on the current sensor signal and resistance of the resistive heating element.

28. The heating system of Claim 21 , wherein the AC power connection is one of a nominal 120VAC service and a nominal 240VAC service.AMENDED SHEET (ARTICLE 19)29. A water heating system for a bathing installation, wherein the bathing installation is one of a spa, swim spa or whirlpool bath, comprising:a resistive heating element disposed in a housing configured for placement in a water flow path of the bathing installation;a switching system configured to selectively connect the resistive heating element to main lines of an AC power connection providing a high voltage AC signal;a heater control system responsive to control signals received via a control interface to control the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontroller configured to control the switching system to control current flow to the heating element; anda monitoring circuit for monitoring the impedance of the heating element; wherein the microcontroller is responsive to the monitoring circuit to detect dry fire conditions in dependence on changes in the impedance of the heating element.

30. The system of Claim 29, wherein the monitoring circuit includes an oscillator circuit coupled to the resistive heating element to provide an oscillator signal to the resistive heating element, and operable even while the resistive heating element is energized, wherein the frequency of the oscillator circuit varies with resistance of the resistive heating element.31.The system of Claim 30, wherein the oscillator circuit is coupled to the resistive heating element by a high pass filter configured to attenuate the high voltage AC signal while allowing the oscillator signal to pass through the filter.

32. The system of Claim 29, wherein the control system is configured to correlate the impedance of the resistive heating element to rapid temperature increase and higher maximum temperature, signaling a dry fire condition.

33. The system of Claim 29, wherein the heater control system is configured to shut off the resistive heater element in the event of a dry fire condition to prevent damage.

34. A water heating system for a bathing installation, wherein the bathing installation is one of a spa, swim spa or whirlpool bath, comprising:a main resistive heating element disposed in a housing configured for placement in a water flow path of the bathing installation, and configured to heat the entire volume of water in the housing;a switching system configured to selectively connect the resistive heating element to main lines of an AC power connection;a heater control system responsive to control signals received via a control interface to control the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontroller configured to control the switching system to control current flow to the heating element; anda flow measuring system for monitoring water flow through the housing, the flow measuring system including one or more local heating elements for applying heat in a local heating zone within the housing under control of the microcontroller, a local temperature sensor for obtaining temperature readings within the local heating zone and a reference temperature sensor disposed upstream or downstream of the local heating zone within the housing, and wherein the microcontroller is responsive to sensor signals from the local and reference sensors to detect and measure water flow through the housing.

35. The water heating system of Claim 34, wherein the one or more local heating elements are positioned directly within the flow path in the housing to create a controlled temperature rise in the water that is proportional to the time the water spends in the local heating zone, to allow for measurement of the water velocity based on the temperature differential between the heated and non-heated water.

36. The water heating system of Claim 34, wherein the one or more heating elements comprise a resistor.

36. The heating system of Claim 34 wherein the heater control system is configured to operate the main resistive heating element independently of the flow measuring system.AMENDED SHEET (ARTICLE 19)37. A water heating system for a bathing installation, wherein the bathing installation is one of a spa, swim spa or whirlpool bath, comprising:a resistive heating element disposed in a housing configured for placement in a water flow path of the bathing installation;a switching system configured to selectively connect the resistive heating element to main lines of an AC power connection;a heater control system responsive to control signals received via a control interface to control the heating system to heat the water in the bathing installation to a selected water temperature, the control system including a microcontroller configured to control the switching system to control current flow to the heating element; anda liquid flow monitoring system for monitoring water presence and flow through the housing, the flow monitoring system including a self-heating thermistor disposed within a probe mounted in the housing, and an excitation circuit to power the selfheating thermistor, wherein the microcomputer is configured to process a thermistor measurement signal to detect flow through the housing.

38. The water heating system of Claim 37, further comprising a second thermistor mounted within the probe, a second excitation circuit to power the second thermistor to send a second measurement signal to the microcomputer, providing the actual water temperature within the heater tube.

39. The water heating system of Claim 38, wherein the respective thermistors are mounted in thermally conductive epoxy within the probe.

40. The water heating system of Claim 38, further comprising a self-heating thermistor flow sensor circuit including the self-heating thermistor, a thermistor temperature sensor circuit including the second thermistor, and a comparator circuit responsive to the outputs of the flow sensor circuit and the temperature sensor circuit to indicate whether flow is detected.

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